FIELD OF THE INVENTION
The invention relates to a method for starting up a continuous-flow steam generator including a combustion chamber having a number of burners for a fossil fuel and a gas-tight containment wall formed from at least approximately vertically disposed evaporator tubes through which a flow passes upwards from below on the feed-water side. The invention also relates to a start-up system for carrying out the method.
In a continuous-flow steam generator, the heating of vertically disposed tubes of an evaporator, which form a gas-tight containment wall of a combustion chamber, leads to a complete evaporation of a flow medium in the evaporator tubes in one pass. Conventionally, during start-up, a circulating stream is superposed on the continuous stream through the evaporator and frequently also through a preheater or an economizer disposed in the continuous-flow steam generator and is heated by flue gas. As a result, the tubes are to be reliably cooled through the use of correspondingly high velocities in the tubes. In that case, with regard to vertically disposed tubes in the containment walls of the combustion chamber, a minimum stream including the continuous stream and the superposed circulating stream is between 25% and 50% of the full-load stream. Accordingly, during start-up, the steam generator load first has to be increased to at least 25% to 50% before reaching continuous-flow operation that is advantageous in terms of efficiency, with its high steam outlet temperatures.
Therefore, as is known from Published European Patent Application 0 054 601 A1, a quantity of flow medium to be conveyed by a feed pump is conventionally preferably kept constant for the start-up and in a load range lying below a specific limit load of 50% of the full load. In that case, the conveying stream of the feed pump is equal to the evaporator throughput. In that mode of operation, the start-up times commencing with the ignition of a first burner of the continuous-flow steam generator and terminating when continuous-flow operation with its high steam temperatures is reached, are very long. That results in relatively high start-up losses since their magnitude is influenced essentially by the start-up times.
High start-up losses also arise from a water excess. That occurs, on one hand, as a result of a mass water flow which is high in comparison with the heat supplied and, on the other hand, as a result of the so-called water ejection. The latter in turn occurs when evaporation commences in the middle of the evaporator and pushes out the water quantity that is present downstream (water plug). Consequently, a continuous-flow steam generator conventionally has a separation device, out of which excess water is drawn off and either supplied again to the evaporator through the use of a circulating pump or discarded. The end of evaporation is thus fixed in that separation device during start-up. A start-up system having a separation device of that type and an additionally required separating vessel, valves and a circulating pump, along with the high technical outlay, necessitate high investment costs which increase sharply when the implementation of high and very high steam pressures is desired.